As the related art relating to this kind of particle beam irradiation method and particle beam irradiation apparatus, the following two papers are known. The first paper is the paper titled “Instrumentation for treatment of cancer using proton and light-ion beams”, by W. T. Chu, et al., printed in the magazine “Review of Scientific Instruments”, 64(8), pp. 2055 to 2096, issued on August 1993.
The second paper is the paper titled “The 200-MeV proton therapy project at the Paul Schrrer Institute: Conceptual design and practical realization”, by E. Pedoroni, et al., printed in the magazine “Medical Physics”, 22(1), pp. 37-53, issued on January 1995.
The first paper describes that in the case where various radiation beams are transformed into thin diameter beams called pencil beams and are irradiated to a human body, the dose distributions of the radiation beams in the body are changed as shown in FIG. 1. As shown in FIG. 1, among various radiations, a radiation beam with a small mass, such as an X-ray or a gamma ray, has a relative dose which becomes maximum in a portion close to the surface of the body, and is decreased as the depth from the surface of the body is increased. On the other hand, a particle beam with a large mass, such as a proton beam or a carbon beam, has a relative dose which has a peak value at a position where the beam stops at a deep portion from the surface of the body, that is, immediately before the range of the particle beam. This peak value is called the Bragg Peak (BP).
A particle beam cancer treatment method is such that this Bragg peak BP is irradiated to a tumor formed in a human organ and the treatment of the cancer is performed. In addition to the cancer, it can also be used for a case where a deep portion of a body is treated. A region to be treated, including a tumor, is generally called an irradiation target. The position of the Bragg peak is determined by the energy of an irradiated particle beam, and as the energy of the particle beam becomes high, the Bragg peak BP is formed at a deep position. In the particle beam treatment, it is necessary that the particle beam is made to have a uniform dose distribution over the whole of the irradiation target to be irradiated. In order to give the Bragg peak BP to the whole area of the irradiation target, “spread of irradiation field (radiation field)” of the particle beam is performed.
This “spread of irradiation field” is performed in three directions of an X-axis, a Y-axis and a Z axis perpendicular to each other. When the irradiation direction of the particle beam is made the direction of the Z-axis, the “spread of irradiation field” is first performed in the Z-axis direction. The “spread of irradiation field” in the irradiation direction of the radiation beam is generally called depth direction irradiation field spread. The second “spread of irradiation field” is such that the irradiation field spread is performed in the X-axis and Y-axis directions, and since the irradiation field spread is performed in the lateral direction perpendicular to the depth direction, it is generally called lateral direction irradiation field spread.
The depth direction irradiation field spread is performed to spread the Bragg peak, which is in the irradiation direction of the particle beam, in the depth direction since the width of the Bragg peak BP in the irradiation direction of the particle beam is narrow as compared with the extent of the irradiation target in the depth direction. On the other hand, the lateral direction irradiation field spread is performed to spread the Bragg peak BP in the direction perpendicular to the irradiation direction since the diameter of the particle beam is smaller than the size of the irradiation target in the direction perpendicular to the irradiation direction. With respect to the depth direction irradiation field spread and the lateral direction irradiation field spread, methods described in the respective foregoing papers will be described.
First, the lateral direction irradiation field spread includes a passive lateral direction irradiation field spread method and an active lateral direction irradiation field spread method. The passive lateral direction irradiation field spread method is a method in which in a particle beam irradiation part of a particle beam irradiation apparatus, a particle beam is irradiated to a scatterer to cause the particle beam to have an extent in the lateral direction, and a uniform dose portion of the center portion is cut out and is irradiated to the target region. In the case where the uniform dose portion can not be made sufficiently large by one scatterer, there is a case where the uniform dose portion is spread by two scatterers, and this is called a double scatterer method. Besides, there is also a method in which two deflection electromagnets provided at the upstream portion of a particle beam irradiation part of a particle beam irradiation apparatus are used to scan the particle beam in a doughnut shape, and the particle beam scanned in the doughnut shape is irradiated to the scatterer to spread the lateral direction irradiation field, and this is called a Wobbler System.
As the active lateral direction irradiation field spread method, there is a method in which a deflection electromagnet provided at the upstream portion of a particle beam irradiation part of a particle beam irradiation apparatus is used to scan the particle beam in the XY plane, and the irradiation position of the particle beam is moved with the lapse of time to obtain a wide irradiation field. In this method, a uniform dose distribution can be obtained by suitably overlapping adjacent irradiation spots of thin diameter pencil beams. Scanning methods of pencil beams include a raster method of performing scanning continuously with respect to time, and a spot method of performing a step-like scanning with respect to time. Incidentally, in this method, although the particle beam is generally called a pencil beam having a thin diameter and is directly irradiated to the target region, there is also a case where the diameter of the pencil beam is slightly enlarged by using a thin scatterer.
Next, the depth direction irradiation field spread will be described. As described before, the width of the Bragg peak BP in the irradiation direction of the particle beam is narrow, and the width of the Bragg peak BP in the irradiation direction is spread by the depth direction irradiation field spread. The Bragg peak BP in which the width in the irradiation direction is spread is called spread-out Bragg peak. First, the depth direction passive irradiation field spread method includes a method in which a comb-type energy modulator called a ridge filter or a range modulator is inserted so as to cross the particle beam.
In both the ridge filter and the range modulator, the thickness of the material of the energy modulator is modulated in the irradiation direction of the particle beam. The ridge filter or the range module decreases the energy of the particle beam according to the modulated thickness, and changes the energy according to the modulated thickness, and consequently, the particle beam in which various energies with different intensities are mixed is irradiated to the irradiation target. Since the range of the particle beam is changed according to the intensity of the energy, the particle beam having various ranges can be irradiated to the irradiation target. In the passive depth direction irradiation field spread method as stated above, the spread-out Bragg peak SOBP in which the width is spread in the irradiation direction can be obtained.
However, the width of the spread-out Bragg peak SOBP is constant in the lateral direction, that is, in the directions of the X and Y axes perpendicular to the irradiation direction of the particle beam, and it can not be changed.
As another depth direction passive irradiation field spread method, there is a method in which a compensator called a bolus is used. In general, a region to be treated in a patient is positioned at the maximum depth of an affected organ in the depth direction, that is, at the deepest part (boundary of the affected organ in the depth direction) of the affected organ in the Z-axis direction, and in general, the depth of the region to be treated has dependency in the lateral direction (X, Y-axis direction), and is changed in the X-axis and Y-axis directions. The change shape of the region to be treated in the depth direction is called a distal shape. As shown in FIG. 2, the bolus BL is an energy modulator which is fabricated for each patient in conformity with this distal shape, and is formed by using polyethylene or wax. By using the bolus BL, while the uniform irradiation dose is irradiated to the X, Y plane, the Bragg peak BP can be conformed to the distal shape.
FIG. 2(a) shows an irradiation target TV and a bolus BL. The irradiation target TV has the deepest layer TVd, and the shape of the deepest layer TVd is called the distal shape. Seven arrows indicate typical particle beams. In FIG. 2(b), the doses of the seven typical particle beams to the irradiation target TV are indicated by a to g. By using the bolus BL, the dose distribution in the deepest layer TVd can be flattened.
As the depth direction active irradiation field spread method, there is a method in which the energy of the particle beam itself irradiated from a particle beam irradiation apparatus is controlled, while the foregoing energy modulator is not used. In this method, the energy of the particle beam is controlled by changing the acceleration energy of an accelerator to accelerate the particle beam, or the energy of the particle beam is changed by inserting a tool called a range shifter so as to cross the particle beam. There is also a method in which both the control of the accelerator and the range filter are used.
In the depth direction active irradiation field spread method, the particle beam is made the beam having the energy of specified intensity, and after the Bragg peak BP with a uniform dose is irradiated to one irradiation layer of the irradiation target, the energy of the particle beam is changed, and the Bragg peak BP is irradiated to an irradiation layer next to the irradiation target TV. Such operation is repeated plural times, and the Bragg peak BP of the particle beam is irradiated to the plural irradiation layers, so that the spread-out Bragg peak SOBP having a desired width in the beam irradiation direction can be obtained. The depth direction active irradiation field spread method is a method in which in the state where the particle beam is not moved in the X and Y-axis directions and is fixed to a definite irradiation position, the energy of the particle beam is changed.
In order to obtain the spread-out Bragg peak SOBP having the desired width, it is necessary to suitably adjust the dose of each irradiation layer of the irradiation target TV, and the dose given to each layer is called “weighting of layer”. This “weighting of layer” is calculated by the same method as the ridge filter or the range module. FIG. 3 shows an example of the dose distribution in the depth direction and the “weighting of layer”. In FIG. 3, the vertical axis indicates the relative dose, and the horizontal axis indicates the depth in the body. A curved line indicated by a solid line indicates calculated values, and plural small squares indicate actually measured values. Plural straight lines extending in the vertical direction indicate the weightings in the respective irradiation layers. This example is a typical example, and the “weighting of layer” is highest at the deepest part. When the weighting of the deepest part is 100, the weighting of the layer adjacent thereto is almost 10 or less.
A particle beam irradiation method in which the depth direction active irradiation field spread method and the lateral direction active irradiation field spread method are combined is described as a spot scanning technique on page 39 to page 45 of the second document.
According to the spot scanning technique, since the energy of the particle beam can be controlled according to the movement of the particle beam in the lateral direction (X, Y-axis direction), the width of the spread-out Bragg peak SOBP in the irradiation direction can also be changed in the lateral direction. Besides, since the energy of the particle beam can also be changed so that the range of the particle beam conforms to the distal shape of a region to be treated, a bolus is not used in the spot scanning technique.
Non-patent document 1: paper titled “Instrumentation for treatment of cancer using proton and light-ion beams”, by W. T. Chu, et al., printed in the magazine “Review of Scientific Instruments”, 64(8), pp. 2055 to 2096, issued on August 1993.
Non-patent document 2: paper titled “The 200-MeV proton therapy project at the Paul Schrrer Institute: Conceptual design and practical realization”, by E. Pedoroni, et al., printed in the magazine “Medical Physics”, 22(1), pp. 37-53, issued on January 1995.